1. Field of the Invention
The present invention relates to a screw attachment, a fastening assembly having the screw attachment, and a process of releasing the fastening assembly, which are effectively applied to an instrument or machine which can be recycled such as an air conditioner, a refrigerator, a television, a battery pack, and a convenient charger.
2. Background Art
Recycling of electric appliances such as an air conditioner, a washing machine, and a refrigerator has been positively realized for reuse of natural resources in view of environmental preservation. However, it takes time to remove securing screws of the electric appliances. Thus, a recycling process with sufficient work efficiency has not been established yet.
FIGS. 16 to 18 show examples of fastening pieces and screw attachments which are made of shape memory alloys. A first prior art as disclosed in Japanese Utility Model Laid-open No. H-2-36616 (particularly in Pages 4 to 6 and FIG. 5 of the prior art) shows a device for preventing looseness of a bolt 60 as well as for releasing the bolt 60 when necessary. As shown in FIG. 16, a washer 62 is provided as a screw attachment sandwiched between a head 61 of the bolt 60 and a fastened plate 68. The washer 62 consists of a first ring 63, a second ring 64, an insulator 65 layered between the rings 63 and 64, and a pair of annular insulating seats 66, 67 each disposed over or under the second ring 64. All the members align with one another in an axial direction of the bolt 60. The rings 63 and 64 each are made of a shape memory alloy. Heating the rings 63 and 64 above a temperature of reverse transformation expands the first ring 63 and contracts the second ring 64 vertically in FIG. 16 due to shape memory processes provided for the rings.
That is, the first ring 63 expands vertically above its reverse transformation temperature, so that a bearing surface 61a of the bolt 60 is pressed to increase contact pressure between threads of the bolt 60 and the fastened plate 68. This secures the bolt 60 to the fastened plate 68, preventing looseness of the bolt 60. The second ring 64 contracts vertically at its reverse transformation temperature and above, so that the bearing surface 61a of the bolt 60 is released from pressure to decrease contact pressure between the threads of the bolt 60 and the fastened plate 68. Thereby, the bolt 60 is brought in a loosened state. Accordingly, the first prior art provides the device to enable fastening and loosening of the bolt 60.
FIGS. 17A and 17B show a second prior art as disclosed in Japanese Patent Application Laid-open No. H-7-4407 (in FIGS. 5 and 12 of the prior art). The second prior art discloses a securing screw 70 to easily disassemble electric appliances for scraping. As shown in FIG. 17A, the screw 70 is made of a shape memory material and has a stem 71 and a head 72 contiguous with the stem 71. The stem 71 has a slit 71a at its fore end to divide the fore end in two parts. The two fore ends each have an outer periphery formed with a tapered surface. The tapered surface has threads 71b formed thereon.
FIG. 17A shows the stem 71 having a closed end which corresponds to a memorized shape, while FIG. 17B shows the stem 71 having an end open in a V-shape. The screw 70 is inserted through a through hole 73a of a fastened plate 73 and engages with an internal thread 74a of a main fastening body 74. A spring 75 is provided between a bearing surface 72a of the screw 70 and the fastened plate 73. A resilient force of the spring urges the screw 70 outward to release it from the main fastening body 74 when the screw 70 returns in its memorized shape.
FIG. 18 shows a third prior art as disclosed in Japanese Patent Application Laid-open No. S-62-110013 (in Page 2 and in FIG. 1 of the prior art). The third art discloses a resilient washer 80 as a screw attachment for preventing looseness of a bolt due to thermal expansion. The resilient washer 80 is made of a shape memory alloy having a shape restoring temperature of 5 to 10□. As shown in FIG. 18, the resilient washer 80 has a pair of arms 82, 83 with a cut-off part 81 therebetween. The arms are offset from each other due to a shape memory process.
The resilient washer 80 is formed in a flat shape at a temperature sufficiently lower than its shape transformation point, and the flat resilient washer 80 is sandwiched between the bolt and a securing object or between a nut and the securing object to contact them with a temperature lower than the shape transformation point. After fastening of the bolt, the resilient washer 80 returns in a memorized shape at a temperature higher than the shape transformation point, causing a tensile force in an axial direction of the bolt. This increases contact pressure between threads of the bolt and the securing object to provide a fastening force of the bolt.
In the third prior art, the return-back of the resilient washer 80 due to a shape memory function is larger than a thermal expansion of the bolt. This prevents looseness between the resilient washer 80 and the bolt with a fastening force regardless of the thermal expansion of the bolt.
Furthermore, another prior art described in Japanese Utility Model Laid-open No. H-4-113320 discloses a washer of a shape memory alloy, and Japanese Patent Application Laid-open NO. 2001-59512 discloses a construction of a washer.
However, the conventional fastening pieces and screw attachments described above have following disadvantages to be eliminated.
In the first prior art, the washer 62 is constituted by several parts to have a complex construction. Thus, the washer 62 is difficult in minimization not to be applied to a small screw. Furthermore, an automatic machine like a robot is employed to fasten screws with a given torque during an assembling step of an electric appliance. Therefore, a heating process after fastening of the screws to increase the fastening force like the first prior art will be disadvantageous in work efficiency for the assembling step. Such a heating process step will be difficult for obtaining a constant fastening force, decreasing productivity of the assembling step.
Moreover, the washer 62 can not be applied to a flush head screw broadly used for assembling electric appliances. Because, the washer 62 is applicable only for a screw or the bolt 60 having the bearing surface 61a perpendicular to an axial direction of the bolt.
The screw 70 of the second prior art has the stem 71 with the pair of branched tips to define a clearance therebetween. The branched tips decrease solidity of the screw 70 not to provide a sufficient fastening force. The screw 70 may be disadvantageously loosened due to vibration of the electric appliance. Furthermore, the screw 70 requires a thread formed thereon with a rotating dice, increasing an assembling cost of the electric appliance. Because, the threads need to have a specifically larger depth to keep a sufficient fastening force.
The washer of the third prior art provides an increased fastening force for a bolt to reliably assemble a plastic chassis of an electric appliance. On the contrary, the increased fastening force is disadvantageous when the bolt is removed during disassembling of the electric appliance.
In another prior art disclosed in Japanese Utility Model Laid-open No. H-4-113320, a fastening bolt has a seating surface perpendicular to an axial direction of the bolt. Thus, a strong fastening force provides a larger contact pressure on the searing surface. The larger contact pressure may undesirably prevent a washer form releasing from the bolt when the washer is heated for disassembling of an electric appliance.
In another prior art disclosed in Japanese Patent Application Laid-open NO. 2001-59512, a washer has a pair of arms and a connection piece for the arms. The connection piece forces into a head of a screw and an opposing object to stop turning of the screw to prevent loosening of the screw. Meanwhile, such a connection piece is undesirable for easily disassembling an electric appliance, because the screw having the connection piece is not easily removed.